1. Field of the Invention
The present invention relates to a method for forming an insulating layer in a semiconductor device, and more specifically, to a method for forming an insulating layer in an NAND flash memory device wherein the insulating layer is formed by means of a STI (shallow trench isolation) process.
2. Discussion of Related Art
As a minimum line width in a next-generation high-integration semiconductor device becomes narrow, the distance of a trench type insulating layer for isolation is narrowed. If a trench is filled with an oxide film, preferably a high-density plasma oxide film by means of an existing single process, voids 10 are generated in the insulating layer, as shown in FIG. 1. In order to solve this problem, the trench is filled in such a manner that a first oxide film 21 is formed, a blanket etch is performed and a second oxide film 22 is then formed, as shown in FIG. 2. However, in the etch process of the first oxide film 21, impurities remain on the first oxide film 21 and are diffused into the device in a subsequent process. This causes the device to degrade.
A case where this insulating layer formation process is applied to a process of fabricating an NAND type flash memory device will now be described in short with reference to FIG. 3A and FIG. 3B.
Referring to FIG. 3A, a tunnel oxide film 32, a polysilicon film 33 and a nitride film 34 are sequentially formed on a semiconductor substrate 31. The tunnel oxide film 32, the polysilicon film 33 and the nitride film 34 are then patterned by means of a lithography process and an etch process using an isolation mask, thus exposing a given region of the semiconductor substrate 31. The exposed semiconductor substrate 31 is then etch to a predetermined depth to form a trench. After a sidewall oxide film is formed in the trench, a first oxide film 35 is formed to fill the trench. Next, the first oxide film 35 is etched using a gas containing fluorine (F). In the etch process of the first oxide film 35, however, an impurity by fluorine contained in the etch gas, for example, a fluorosilicate glass (FSG) film 36 is generated on the first oxide film 35.
By reference to FIG. 3B, a second oxide film 37 is formed on the entire structure and is then subjected to a blanket etch to form an insulating layer.
However, as the insulating layer formed by the above process is subjected to a subsequent annealing process, fluorine of the FSG film 36 formed on the first oxide film 35 is diffused into the device. This can degrade the characteristics of the device.